Linear accelerator having the beam injected at a position of maximum r.f. accelerating field



Dec.'8 ,'l970 Filed NOV. 24, 1967 P. G. STARK LINEAR ACCELERATOR HAVING THE BEAM INJECTED AT A OF MAXIMUM R.F. ACCELERATING FIELD POSITION V 2 Sheets-Sheet 1 lllnvI/llllm FIG. 2 PRIOR ART INVENTOR.

PETER G. STARK ATTORNEY Dec. 8, 1970 P; G STARK 3,546,524

LINEAR ACCELERATOR HAVING 'EHE BEAM INJECTED AT A POSITION OF MAXIMUM R.F. ACCELERATING FIELD Filed Nov. 24, 1967 2 Sheets-Sheet 2 N 3 "/2 MODE FIG-4 A i i I L O \Q 0 ill-AM F} {a Q 5 FIG.5 A 1 o 0 0 U 0 5'5, 1 Ia His EH8 la la BEAM 5 A i i BEAM :1 A

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PETER G. STARK BY j sli fi F M ATTORNEY United States Patent 3,546,524 LINEAR ACCELERATOR HAVING THE BEAM INJECTED AT A POSITION 0F MAXIMUM R.F. ACCELERATING FIELD Peter G. Stark, Menlo Park, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Nov. 24, 1967, Ser. No. 685,466 Int. Cl. H01j 25/10 US. Cl. 315-541 9 Claims ABSTRACT OF THE DISCLOSURE erator section, into which the charged particles are in-' jected is formed and arranged such that the injected beam enters the accelerating microwave field of the first cavity at a position of nearly maximum electric field of the spatial distribution of the electric field. In this manner, subsequent charged particles injected into the electric field of the resonator, within the acceptance angle of the time.- varying electric field over which the accelerating field is increasing with time, tend to be more rapidly accelerated, thereby forming a tighter bunch of charged particles than would otherwise be obtained if the particles were not injected into a position in the cavity of maximum electric field. In addition, this method of injection permits the charged particles to be injected into the first cavity with initial velocities corresponding to 0 kv. beam voltage on up to much higher initial velocities, whereby a relatively inexpensive beam injection gun may be employed and hereby the output beam current of the accelerator may be easily controlled by varying the anode voltage on the beam injection gun.

DESCRIPTION OF THE PRIOR ART Heretofore, microwave linear accelerators have been constructed of the standing wave type employing a plurality of cavity resonators coupled together and successively arranged along the beam for accelerating the beam particles to nearly the velocity of light. In such prior accelerators the beam was injected into the first upstream cavity at a position of less than maximum electric field of the spatial distribution of field within the cavity. More specifically, the spatial point of maximum electric field within the cavtiy occurred at a position downstream of the point at which the beam was injected into the cavity. As a result, a first beam particle entering the cavity, within the acceptance angle of the time-varying electric field, experienced a time-varying increasing accelerating electric field, which increased not only due to the time-varying nature of the electric field but also due to the fact that the spatial maximum of the time-varying electric field occurred at a point downstream of the point of injection. As a consequence, the spatial distribution of the electric field within the cavity produced a debunching effect since subsequent charged particles entering, within the acceptance angle of the time-varying electric field, were not as tightly bunched with the first particles, since the first particles were experiencing a stronger electric field. due to the spatial distribution, tending to pull them away from the second particles.

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The effect of the spatial distribution of the accelerating field within the beam injection cavity was largely overcome by employing relatively high beam injection velocities, such as beam injection voltages on the order of or in excess of kv. Such relatively high beam injection voltages require a relatively expensive beam gun which is to be avoided if possible. Moreover, when the beam injection voltage is on the order of 100 kv., control of the output power of the accelerator is not readily controlled by controlling the beam voltage of the beam injection gun. Therefore, a need exists for an improved beam injection scheme which will permit a relatively low beam injection voltage to be employed, and which will readily permit control of the output current of the accelerator by controlling the input beam voltage.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved linear particle accelerator.

One feature of the present invention is the provision, in a microwave linear particle accelerator, of a beam injection cavity arranged such that the beam is injected into the cavity at a point of nearly maximum electric field distribution within the gap of the cavity, whereby improved beam bunching is obtained and whereby relatively low beam injection voltages may be employed.

Another feature of the present invention is the same as the preceding feature wherein the beam entrance hole into the beam injection cavity is substantially smaller than its beam exit hole, whereby the beam enters the cavity at a point of maximum electric field of the cavity.

Another feature of the present invention is the same as any on or more of the preceding features wherein the upstream end wall of the beam injection cavity has a planar inside surface forming a reflective plane in the cavity and the axial length of the cavity is approximately half of the axial length of successive accelerating cavities to facilitate injection of the beam at a point of maximum electric field within the cavity.

Another feature of the present invention is the same as any one or more of the preceding features wherein the beam entrance wall of the beam injection cavity forms the accelerating electrode of the beam injection gun, and means for varying the beam current by varying the potential applied between a source ofbeam particles and the accelerating electrode is included, whereby the output beam power of the accelerator is readily varied.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal view, partly in section and partly in block diagram form, of a microwave linear accelerator incorporating features of the present invention,

FIG. 2 is a perspective view, partly in cross-section, of the first portion of a prior art accelerator,

FIG. 3 is a schematic line diagram of a prior art discloaded accelerator section depicting the 7r/2 mode of operation therein,

FIG. 4 is an w-B diagram for a resonant microwave accelerator section,

FIG. 5 is a schematic line diagram showing the electric field pattern of a prior art accelerator section operating in the 1r/2 mode.

FIG. 6 is a plot of electric field intensity vs. distance across the gap of the first cavity of a prior art accelerator,

FIG. 7 is a sectional view of that portion of FIG. 1 delineated by line 7-7 and including a plot of electric field intensity vs. distance across the gap of the beam injection cavity of the present invention,

FIG. 8 is a plot of time-varying accelerating field vs. time in the gap of a beam injection cavity, and

FIG. 9 is a plot of electron energy vs. distance along a bunch of electrons, depicting the bunching of the prior art and that obtained by employing features of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 there is shown a microwave linear particle accelerator 1 incorporating features of the present invention. The accelerator 1 includes an accelerating section 2 having a plurality of cavity resonators 3 successively arranged along a beam path 4 for electromagnetic interaction with charged particles within the beam for accelerating the charged particles to nearly the velocity of light at the downstream end of the accelerator section 2. A source of beam particles such as a charged particle gun 5 is disposed at the upstream end of the accelerator section 2 for forming and projecting a beam of charged particles, as of electrons, into the accelerator Section 2. A beam output window 6, which is permeable to the high energy 'beam particles and impermeable to gas, is sealed across the downstream end of the accelerator section 2. A suitable output beam window 6 comprises a thin aluminum foil. The accelerator section 2 and the gun 5 are evacuated to a suitably low pressure as of l torr by means of a high vacuum pump 7 connected into the accelerator section 2 by means of an exhaust tubulation 8.

The accelerator section 2 is excited with microwave energy from a microwave source 9, such as a klystron amplifier, connected into essentially the center of the accelerator section 2 by means of a waveguide 11 having a microwave window 12 sealed thereacross to permit passage of microwave energy into the accelerator while forming a portion of the vacuum envelope of the accelerator section 2. The accelerator section 2 is a standing wave accelerator, i.e., a resonant section of coupled cavities, resonant at S-band, and the microwave source 9 delivers approximately 1.6 megawatts of S-band power to the accelerator section 2. The resonant microwave fields of the accelerator section 2 electromagnetically interact with the charged particles of the beam 4 to accelerate the particles to essentially the velocity of light at the downstream end of the accelerator. More particularly, the 1.6 megawatts of input microwave power produce output electrons in the beam 4 having energies of approximately 4 m.e.v. These high energy electrons may be utilized to bombard a target to produce high energy X-rays or, alternatively, the high energy electrons may be employed for directly irradiating objects, as desired.

Referring now to FIG. 2 there is shown a prior art accelerator section 2. More specifically, the prior art accelerator section 2 includes a plurality of interaction cavities 3 successively arranged along the beam path 4 for electromagnetic interaction with the beam to accelerate the beam particles to nearly the velocity of light. A plurality of coupling cavities 15 are disposed off the axis of the accelerator section 2 for electromagnetically coupling adjacent interaction cavities 3. Each of the coupling cavities 15 include a cylindrical side wall 16 and a pair of centrally disposed inwardly projecting capacitive loading members 17 projecting into the cylindrical cavity from opposite end walls thereof to capacitively load the cavity. Each cylindrical coupling cavity 15 is disposed such that it is approximately tangent to the interaction cavities 3 with the corners of each coupling cavi y 15 intersecting the inside walls of the interaction cavities 3 to define inductive coupling irises 18 providing wave energy communication between the interaction cavities 3 and the associated coupling cavity 15. The interaction cavities 3 and the coupling cavities l are all tuned to essentially the same frequency.

Referring now to FIG. 3 there is shown, in line diagram form, a disc-loaded accelerator section. The coupled cavity accelerator section 2 of FIG. 2 can be considered a modified form of the disc-loaded accelerator section of FIG. 3. More particularly, the disc-loaded structure of FIG. 3 has an w-fi diagram of the type indicated in FIG. 4. If there are six cells in the disc-loaded waveguide, and the disc-loaded waveguide is resonant, the w-fi? diagram is split into a number of discrete operating points indicated by the black dots on the w-{i diagram. A particularly desirable accelerating mode of operation is the 1r/ 2 mode of the disc-loaded waveguide structure wherein the electric field configuration is indicated by the arrows in FIG. 3. In the 1r/2 mode, strong resonant microwave fields at any given instant of time are separated by of phase shift in adjacent resonators such that when the microwave field is at maximum in the first resonator, it is at a null in the second resonator and at a peak negative direction in the third resonator and at a null in the fourth resonator, etc. In the accelerating mode, the cavities having a node of a zero field intensity contribute essentially nothing to the acceleration of the beam of particles. Therefore, the disc-loaded structure of FIG. 3 is modified as indicated in FIG. 5 by moving the nodal cavities off the axis of the beam such that they serve as coupling cavities between adjacent interaction cavities 3. Moreover, the interaction cavities have been modified in shape to a generally toroidal shape to increase their Q. By moving the coupling cavities 15 off the axis of the beam and by increasing the Q of the interaction cavities, the interaction efiiciency between the accelerating fields of the periodic accelerator section and the beam is substantially increased, thereby permitting the accelerator section to be substantially shortened for a given maximum energy of the electrons exiting from the accelerator section, as compared to the length of a disc-loaded waveguide structure of FIG. 3.

One of the problems with the prior art coupled cavity accelerator section 2 of FIG. 2 is that the beam, for proper bunching, must be injected into the first cavity with a velocity corresponding to a beam voltage of approximately kv. The reason why the beam voltage has to be relatively high is seen with regard to FIG. 6, which shows that the electric field intensity, for the first accelerating cavity, has a spatial distribution such that the maximum ccelerating electric field is encountered at approximately the mid-plane of the cavity 3. In such a case, a first electron entering the first interaction gap sees an increasing accelerating field due to the spatial distribution of the electric field in the gap. This spatial distribution of the field tends to accelerate a first electron entering the gap away from a second electron subsequently entering the gap within the acceptance angle of the time-varying electric field, which typically occurs between 0 and of the time varying electric field in the gap, as indicated in FIG. 8. The time-varying character of the electric field in the gap tends to offset the debunching effect of the spatial variation because a subsequent electron entering the gap sees a stronger accelerating electric field due to the time-varying character of the field. However, the spatial distribution of the field tends to detract from optimum bunching of the beam, for the reasons as aforedescri'bed. The high injection velocities for the beam tend to counteract the debunching effect due to the spatial distribution of the electron field. Therefore, the relatively high injection beam velocities are required for the prior art accelerator section having a full-sized upstream beam injection cavity.

Referring now to FIGS. 1 and 7 there is shown the injection cavity design according to the teachings of the present invention. In the present invention, the injection cavity 3' is arranged such that the spatial distribution of the accelerating electric field in the gap of the first cavity 3 has a maximum intensity at the point at which the electron beam enters the cavity. As a result, a subsequent electron entering the gap within the acceptance angle of the time-varying electric field sees a maximum intensity accelerating field followed by a decreasing component of the accelerating field due to spatial distribution. More specifically, as the electron moves across the gap the spatial distribution of the accelerating field causes the preceding electrons to see a weaker accelerating field and, therefore, to be more strongly controlled by the timevarying component of the electric field which tends to more rapidly accelerate the subsequent electrons to thereby form a tighter bunch of electrons. Thus, the debunching effect caused by the prior art spatial distribution of the accelerating electric field across the gap is overcome in the present invention because the electrons are injected into a region of maximum electric field within the gap of the injection cavity 3'.

One convenient way of forming a cavity having an electric field distribution in the gap which decreases from the point of injection as indicated in FIG. 7 is to make the upstream end wall of the injection cavity 3' approximate a reflecting plane disposed midway of the length of the standard cavity. If the shorting plane is placed midway along the length of a standard cavity, the half-sized cavity has the same resonant frequency as the full-sized cavity and, moreover, the electric field distribution is now a1- tered such that the spatial distribution of electric field is a maximum at the plane of the shorting plane. Thus, in the beam injection cavity 3' of FIG. 1, the upstream end wall 21 serves as a reflecting plane for a one-half sized cavity and the one-half sized cavity has the same resonant frequency as the remaining full-sized cavities 3. In order for the upstream end wall 21 to approximate as closely as possible a reflecting plane, the beam entrance hole 22 should be as small as possible. More particularly, the beam entrance hole 22 should have minimum characteristic transverse dimension, i.e., diameter, substantially less than the diameter of the beam exit hole 23 which is disposed in the downstream end wall 24 of the beam injection cavity 3. In one example of an S-band beam injection cavity 3' of the present invention, the beam entrance hole 22 had a minimum diameter of 2 mm. and the beam exit hole 23 had a minimum diameter of 10 mm.

When the beam is injected into the injection cavity 3' at a point of maximum spatial distribution of the accelerating electric field, the beam may be injected at very low initial velocities substantially varying from volts to 40 kv. or above. The ability to use very low beam injection voltages or velocities is especially desirable as it permits a great simplification of the beam injection gun 5. More particularly, it permits the centrally apertured beam entrance end wall 21 to form the accelerating electrode of the beam gun and permits relatively low voltage insulators to be employed between the source (cathode) 25 and the accelerating electrode (anode) 21 of the gun 5. It also readily permits the beam current to be varied by varying the potential between the source 25 and the accelerating electrode 21 of the gun 5, as indicated by variable potential source 28 of FIG. 7. It also permits higher beam currents with less transverse defocusing of its beam than heretofore obtained for a given gun design,

Another advantage of being able to inject the beam at relatively low beam voltages is that the bunching of the beam by the injection cavity is accomplished in a relatively short distance along the beam path due to the relatively low velocity of the electrons. When the electrons have relatively high injection beam voltages, i.e., in excess of 75 kv. the bunching mechanism requires a substantial length of the beam path which necessitates changes in the phase velocity of the accelerating wave along the beam path thereby complicating the design of the accelerator.

In a typical S-band accelerator having the geometry as shown in FIG. 1, which employs the half-sized beam injection cavity 3, the electrons were injected into the injection cavity 3 at approximately 40 kv. With approximately 1.6 megawatts of S-band power applied to the resonant accelerator section 2, the output beam of electrons had bunches as indicated by curve 27 of FIG. 9, wherein approximately 60% of the beam current was closely grouped to electron energies of approximately 4 m.e.v. with a beam output current of approximately milliamps. When using the prior art accelerator section 2 of FIG. 2, with the same beam injection voltage, the output beam bunches were as indicated by the prior art line of FIG. 9, wherein approximately 10% of the beam current was grouped with electron energies in the range of 2 to 3 m.e.v.

Thus, it is seen that the provision of injecting the electron beam at a point of maximum electric field distribution within the beam injection cavity 3 results in a substantial improvement in the bunching of the electron beam and greatly increases the efiiciency of the accelerator section 2. Moreover, it permits the use of an inexpensive electron gun and allows the anode to cathode voltage applied to the electron gun to control the output beam current.

Although a preferred embodiment of the present invention has the upstream end wall 21 of the beam injection cavity 3' disposed at a plane of symmetry as compared to the remaining cavities 3 to form a half-length cavity, this is not a requirement. The beam injection cavity may take other forms, wherein the beam injection cavity is dimensioned for resonance at the same frequency as the other cavities 3 and the axial length of the beam injection cavity is not necessarily equal to one-half the length of the other cavities.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention can be made without departing from the scope thereof it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a standing wave linear accelerator, means for forming a beam of charged particles to be accelerated, a resonant accelerator section disposed along the path of the beam and having a plurality of coupled cavity resonators successively disposed along the beam path, means for introducing power into said accelerator section for forming a standing wave electromagnetic field extending throughout said section including the first upstream cavity resonator of said section for electromagnetic interaction of the electric fields within said cavity resonators with the beam to accelerate the charged particles thereof, the first upstream cavity resonator of said accelerator section being shaped to provide at the point of entry of said beam into said first cavity a position of nearly maximum electric field intensity of the spatial distribution of the resonant field of said first cavity resonator during the acceptance angle of said field whereby both improved bunch ing of the charged particles and acceleration of the beam is obtained in said first cavity.

2. The apparatus of claim 1 wherein the axial spacing between the upstream end wall and the downstream end wall of said first upstream cavity is substantially less than the axial spacing between the end walls of a plurality of said interaction cavities successively disposed downstream of said first cavity.

3. The apparatus of claim 4 wherein said upstream end wall has a substantially planar surface facing said downstream end wall, whereby said upstream end wall serves as a reflecting plane in said cavity resonator to provide a nearly maximum electric field at the point of beam injection into said upstream cavity.

4. The apparatus of claim 3 wherein said upstream end wall structure has an axially directed beam hole through which the beam passes into said first cavity resonator and said downstream end wall has a beam exit hole through which the beam exits from the first cavity to the next successive beam interaction cavity, the beam hole in said upstream end wall having a smaller characteristic transverse dimension than the beam exit hole in said downstream end Wall.

5. The apparatus of claim 4 wherein said axial spacing of said end walls of said first cavity is approximately onehalf of the axial spacing of the end walls of said plurality of interaction cavities disposed successively downstream of said first cavity.

6. The apparatus of claim 1 wherein said means for forming a beam of charged particles includes, means forming a source of charged particles, and said upstream cavity has an apertured upstream end wall forming an accelerating electrode for said source of charged particles.

7. The apparatus of claim 6 including means forming a source of variable potential connected between said source of charged particles and said accelerating electrode for varying the beam current injected into said upstream cavity resonator to vary the output beam current of the accelerator.

8. In a method of accelerating a beam of charged particles by interaction with resonant electric fields of a coupled cavity standing wave accelerator section the steps of, exciting a standing wave electromagnetic field to extend throughout said accelerator section including into the first upstream cavity thereof, injecting the beam of charged particles into the upstream cavity at a position in the upstream cavity of nearly a maximum electric field of the spatial distribution of the accelerating electric field therein, and passing the beam of charged particles through the remainder of said upstream cavity for acceleration therein and successively through the axially directed resonant electric fields of the coupled cavity structure to produce cumulative electromagnetic interaction between the accelerating fields of the cavities and the beam of charged particles.

9. The method of claim 8 including the step of employing the upstream end wall of the upstream cavity as the accelerating electrode of a charged particle gun such that by varying the potential between a source of charged particles and the accelerating electrode of the gun the beam current may be readily varied.

References Cited UNITED STATES PATENTS 2,813,996 11/1957 Chodorow 3155.42 3,011,087 11/1961 Jeppson et al 3155.42 3,147,396 9/1964 Goerz et a1. 3155.42

HERMAN KARL SAALBACH, Primary Examiner S. CHATMON, JR., Assistant Examiner US. Cl. X.R.

p UNITED S'IIA'IES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,546 524 Dated 1970 December 8 Inventor-( Peter G. Stark It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 3, line 1, change "4" to --2--.

Signed and sealed this 11th day of June 1971;..

(SEAL) Atteat:

EDJARD M.FLETGHER,JR. C. MARSHALL DANN Attesting Officer Commissioner of Paton 

